In the production of low molecular weight epoxy resins from polyhydric phenols and epihalohydrins, two reactions must be accomplished: 1) the etherification (coupling) reaction to make the bishalohydrin ether of the polyhydric phenol, and 2) the dehalohydrogenation (epoxidation) reaction of the bishalohydrin ether of the polyhydric phenol with an inorganic hydroxide to make the epoxy resin. Two approaches to this problem are used. In the first, the two reactions are accomplished simultaneously, typically by adding an aqueous inorganic hydroxide mixture (usually 20-50% sodium hydroxide) to an organic mixture containing the polyhydric phenol and the epihalohydrin. In the second approach, a catalyst is used to etherify (couple) the polyhydric phenol and the epihalohydrin to produce the bishalohydrin ether, and then the dehalohydrogenation (epoxidation) reaction is accomplished by adding an aqueous inorganic hydroxide mixture (usually 20-50% sodium hydroxide) to an organic mixture containing the bishalohydrin ether and an organic solvent, where the solvent may be excess epihalohydrin. In both approaches, the challenge is to accomplish these reactions rapidly with equipment that does not require a large capital investment, while also conducting the reactions in such a way that the product meets the desired product specification and raw materials are not lost due to side reactions.
Currently, the most common way to prepare epoxy resins from polyhydric phenols is to perform the two reactions simultaneously in batch reactors by addition of an aqueous inorganic hydroxide mixture (typically 20-50% sodium hydroxide) to a mixture of the polyhydric phenol in excess epihalohydrin. Water may be left in the reactor to form a two-phase mixture or removed by azeotropic distillation to precipitate the inorganic halide salt. In either case, the maximum size of the reactors is limited, so a large facility will contain multiple batch reactors, increasing the cost of the facility. If the water is not removed, epihalohydrin hydrolysis reactions can result in a significant loss of the epihalohydrin to byproducts. If the water is removed, then the solid salt must be handled by downstream equipment, increasing the cost of the plant. When the water is removed, the reaction is also sensitive to excess hydroxide addition, which causes the formation of insoluble polyglycidol-type polymer.
In designing a process to produce epoxy resins from polyhydric phenols and epihalohydrins, the challenge is to identify a reaction scheme which accomplishes the reactions quickly in reactor equipment which does not have high capital costs, to conduct the reaction in such a manner that very little of the epihalohydrin is lost to byproduct reactions and no insoluble polymers are formed, and so that the separations equipment required for the rest of the process also does not have a high capital cost.